The present invention relates to wireless communication, and more particularly, to a control method capable of reducing the call dropped rate of a mobile station in a wireless communication system, and a control circuit and mobile station thereof.
As wireless communication system technology progresses, wireless communication and personal mobile communication have become essential in our daily life nowadays. In a wireless communication system, many different interference sources are provided, such as external interference, interference between signals of the wireless communication system itself, multi-path fading of a signal itself in the wireless communication system, Inter-Symbol Interference (ISI), and so on. Therefore, how to reduce the call dropped rate of a mobile station in the wireless communication system has become an important issue of wireless communication.
According to various standards of wireless communication, such as Global System for Mobile communication (GSM) and General Packet Radio Service (GPRS), a mobile station should monitor receiving power levels of a plurality of neighbor cells near a serving cell in the wireless communication system and maintain the Base Station Identification Codes (BSICs) of six neighbor cells corresponding to the six strongest receiving power levels. Once the wireless communication system instructs the mobile station to switch to a new serving cell being one of the six neighbor cells because of poor communication quality of the serving cell, the mobile station is capable of performing the switch accordingly to avoid call drop problems.
According to definitions of the wireless communication related specifications, while at a communicating state, the mobile station should receive a measurement result of neighbor cells from a lower layer of the mobile station every 0.5 seconds. The mobile station has to determine which six neighbor cells have the six strongest receiving power levels according to the measurement result. If in the neighbor cells corresponding to the six strongest receiving power levels, at least one (i.e. one or more) neighbor cell having unknown BSIC exists, the mobile station has to enter a priority mode M1 to perform frequency correction and synchronization of the mobile station with respect to the at least one neighbor cell's Broadcast Control Channel (BCCH) to decode the at least one neighbor cell's BSIC. In addition, according to definitions of the wireless communication related specifications, the time period in which the mobile station stays in the priority mode M1 is limited to five seconds.
After the mobile station entered the priority mode M1, if the BSICs of six neighbor cells corresponding to the six strongest receiving power levels are known or the time period in which the mobile station stays in the priority mode M1 has reached five seconds, the mobile station has to leave the priority mode M1 and enter a reconfirmation mode M2. In the reconfirmation mode M2, the mobile station will reconfirm the synchronization to all other neighbor cells having known BSICs, wherein the neighbor cells having known BSICs are not necessarily the neighbor cells having the six strongest receiving power levels. When the mobile station completes the reconfirmation of the synchronization to all the neighbor cells having known BSICs, the mobile station has to leave the reconfirmation mode M2. In addition, according to definitions of the wireless communication related specifications, the mobile station has to enter the reconfirmation mode M2 again within ten seconds.
Accordingly, if the communication quality of the serving cell is poor, the wireless communication system will select a neighbor cell with better communication quality out of the neighbor cells having known BSICs and switch the mobile station to the neighbor cell selected by the wireless communication system. That is, the neighbor cell selected by the wireless communication system will be a new serving cell of the mobile station. However, the wireless communication system can select a neighbor cell with better communication quality only out of the neighbor cells having known BSICs. Therefore, if the number of the neighbor cells having known BSICs is limited, the number of selectable neighbor cells by the mobile station is also limited. Therefore, it is very likely that even if the wireless communication system switches the mobile station to a new serving cell, the poor communication quality is not improved. In the worst case, there could be no selectable neighbor cell. As a result, the mobile station suffers from a high call dropped rate.